This invention relates to Nuclear Magnetic Resonance (NMR) Imaging and more particularly, relates to NMR Imaging of materials to determine petrophysical properties of such materials.
Relative permeability, capillary pressure, and residual oil saturation of an earth formation are important properties needed in the reservoir engineering of an oilfield. These properties help a reservoir engineer determine, for example, the productivity of a reservoir, the total reserves, and the future success of various oil recovery processes, such a waterflooding or carbon dioxide flooding.
The oil industry widely uses laboratory core flood experiments to obtain these and any other properties which may be useful for modeling reservoir performance during primary, secondary, and tertiary oil recovery processes. In any core flood experiments, the major difficulty is determining the actual fluid saturations within the core during the flooding process. Many methods have been used for determining fluid saturations during these core flood displacements, including resistivity, microwave absorption, gamma-ray attenuation, neutron radiography, etc. However, these methods often impose restrictions on experimental techniques and may provide values for saturations that are averaged over a substantial volume of the core.
A recent development, X-ray CT, may be used to obtain three-dimensional images of the fluid saturations during displacement processes at reservoir pressures and temperatures; see for example U.S. Patent Application Ser. No. 623,297, filed June 22, 1984. Although X-ray CT is a major advance over other prior art methods, it unfortunately has some inherent restrictions. Since X-ray CT images are cross-sectional slices of finite thicknesses, typically 1-10 mm thick, it is not possible to simultaneously image the entire core. For any rapid displacement processes, this may result in reconstructed images that are not correct. X-ray attenuation and spectral beam hardening are other difficulties often encountered in X-ray CT Imaging.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and improved methods are provided for NMR Imaging of porous materials to measure saturations of fluids inside a sample of such material and to determine petrophysical properties of such material.